Liquid crystal device comprising array of sensor circuits using a pre-charge operation

Abstract

A liquid crystal device is provided, for example in the form of a combined display and sensor forming a touch screen. The device comprises an array, for example of active matrix type, of sensor circuits. Each sensor circuit comprises a liquid crystal sensing capacitor (CV) connected between a transistor M 1 arranged as a source-follower and a precharging input (PRE). A sensor selecting capacitor (C 1 ) is connected between the transistor (M 1 ) and a row select line (RWS).

1 . A liquid crystal device comprising a first array of first sensor circuits, each of which comprises an amplifier, a liquid crystal sensing capacitor connected between an input of the amplifier and a sensor circuit precharge input, and a further capacitor connected between the amplifier input and a sensor circuit selecting input. 
     
     
         2 . A device as claimed in  claim 1 , in which the sensing capacitor comprises a planar capacitor having co-planar electrodes cooperating with an adjacent layer of liquid crystal material. 
     
     
         3 . A device as claimed in  claim 2 , in which the co-planar electrodes face an electrode gap on an opposite side of the layer. 
     
     
         4 . A device as claimed in  claim 2 , in which the co-planar electrodes face an electrically floating electrode on an opposite side of the layer. 
     
     
         5 . A device as claimed in  claim 2 , in which the co-planar electrodes are surrounded by a co-planar guard ring arranged to receive a substantially fixed voltage. 
     
     
         6 . A device as claimed in  claim 1 , in which the sensing capacitor has a capacitance which changes in response to a touch event. 
     
     
         7 . A device as claimed in  claim 1 , in which the precharge input is arranged to receive a first voltage during a first precharge period and a second voltage whose value is less than that of the first voltage during a second precharge period. 
     
     
         8 . A device as claimed in  claim 1 , in which the selecting input is arranged to receive a third voltage for inhibiting the first sensor circuit during an inhibiting period and a fourth voltage whose value is greater than that of the third voltage for enabling the first sensor circuit during an enabling period. 
     
     
         9 . A device as claimed in  claim 7 , in which the selecting input is arranged to receive a third voltage for inhibiting the first sensor circuit during an inhibiting period and a fourth voltage whose value is greater than that of the third voltage for enabling the first sensor circuit during an enabling period, and the enabling period begins during the second precharge period. 
     
     
         10 . A device as claimed in  claim 9 , in which the enabling period and the second precharge period end substantially simultaneously. 
     
     
         11 . A device as claimed in  claim 1 , in which the amplifier comprises a first transistor. 
     
     
         12 . A device as claimed in  claim 11 , in which the first transistor comprises a first metal oxide semiconductor field effect transistor. 
     
     
         13 . A device as claimed in  claim 12 , in which the first transistor is connected as a source-follower. 
     
     
         14 . A device as claimed in  claim 13 , in which the first array comprises rows and columns of the first sensor circuits with the source-followers of each column of the first sensor circuits being connected to a common source load. 
     
     
         15 . A device as claimed in  claim 14 , in which the selecting inputs of the first sensor circuits of each row are connected together. 
     
     
         16 . A device as claimed in  claim 14 , in which the precharge inputs of the first sensor circuits of each row are connected together. 
     
     
         17 . A device as claimed in  claim 1 , in which the further capacitor comprises a voltage dependent capacitor. 
     
     
         18 . A device as claimed in  claim 17 , in which the voltage dependent capacitor comprises a second metal oxide semiconductor field effect transistor. 
     
     
         19 . A device as claimed in  claim 18 , in which a source and drain of the second field effect transistor are connected together. 
     
     
         20 . A device as claimed in  claim 1 , in which each of the first sensor circuits comprises a diode having a first terminal connected to the amplifier input and arranged to provide a predetermined voltage at the amplifier input when the first sensor circuit is inhibited. 
     
     
         21 . A device as claimed in  claim 18 , in which the second field effect transistor has a source-drain path connected between the amplifier input and a first terminal of a diode arranged to provide a predetermined voltage at the amplifier input when the first sensor circuit is inhibited. 
     
     
         22 . A device as claimed in  claim 20 , in which a second terminal of the diode is connected to the precharge input. 
     
     
         23 . A device as claimed in  claim 1 , comprising a second array of liquid crystal display pixels. 
     
     
         24 . A device as claimed in  claim 23 , in which the first and second arrays are addressed by a common active matrix addressing arrangement. 
     
     
         25 . A device as claimed in  claim 24 , in which the addressing arrangement is arranged to address the first array during display blanking periods. 
     
     
         26 . A device as claimed in  claim 23 , in which the first sensor circuits have outputs connected to data input lines connected to pixel data inputs. 
     
     
         27 . A device as claimed in,  claim 23  in which each of the first sensor circuits is associated with a group of at least one of the pixels. 
     
     
         28 . A device as claimed in  claim 27 , in which each group comprises a composite colour group of sub-pixels. 
     
     
         29 . A device as claimed in  claim 1 , comprising a third array of second sensor circuits having sensitivities less than those of the first sensor circuits. 
     
     
         30 . A device as claimed in  claim 29 , in which the second sensor circuits are interleaved with the first sensor circuits. 
     
     
         31 . A device as claimed in  claim 1  arranged to operate as a touch screen.